1. Field of the Invention
The present invention relates to photolithography, more specifically, to an inspection method for an illumination optical system of an exposure tool.
2. Description of the Related Art
Accompanying progress of semiconductor manufacturing technology, more specifically, miniaturization of a semiconductor device and enhancement of integration density, the performance specification required for an exposure tool, which transfers a fine pattern onto a semiconductor substrate, has critical. Since a variation in dimensions of a pattern which configures a semiconductor device, affects the operating speed of the semiconductor device, an exposure process is required to transfer a uniform resist pattern with high precision in a one-shot exposure region.
In the exposure process of photolithography, a photomask on which a mask pattern including a transparent portion, an opaque portion, and a translucent portion is delineated, is uniformly illuminated by illumination beams from an illumination optical system so as to project a mask pattern image onto a one-shot exposure region on a semiconductor substrate using a projection optical system. Since a resist film is coated on the surface of the semiconductor substrate, a resist pattern is formed on the semiconductor substrate by a development process after exposure.
The illumination optical system of the exposure tool includes a fly's eye lens and a condenser lens. The fly's eye lens receives illumination beams from a light source, and forms effective light sources on an output side of the fly's eye lens. The condenser lens gathers the emerged beams from the output side of the fly's eye lens, and uniformly illuminates a region on which the mask pattern of the photomask is delineated.
Due to various characteristics of the illumination optical system of the exposure tool, pattern dimensions that need to be of the same dimension may vary in the one-shot exposure region. A reason for a variation in the pattern dimensions, for example, may be uneven illumination, where intensity of the illumination beams illuminating the photomask (hereinafter, referred to as an “exposure dose”), varies from place to place. When a pattern dimension almost substantially equal to or less than the wavelength of the illumination beam, an exposure with an exposure dose different from an optimum exposure dose causes a variation of a transferred resist pattern dimension. Since the finer the pattern dimension, the smaller the allowable exposure dose range (exposure latitude) for formation of a resist pattern with a desired dimension, the uneven illumination must be strictly controlled. The uneven illumination is caused by a local defect, such as dust or a scratch on a surface of a lens included in the illumination optical system.
Another factor that causes a variation in resist pattern dimensions in a one-shot exposure region is a variation in the shape of an effective light source, in particular, a variation in a size thereof (,, value). Since the ,, value is one of the factors that determine an imaging characteristic of a lens, the variation in the ,, value represents changes in resolution and exposure latitude. In addition, in recent years, a modified illumination such as annular illumination is actively utilized for improving resolution. In this case, in addition to the ,, value, the variation of an annular shield factor in the one-shot exposure region causes a variation of the resist pattern dimension. For example, the ,, value varies due to an aberration of the condenser lens located between the effective light source and the photomask, and the variation of the resist pattern dimension occurs (Proceedings of SPIE, March, 1999, Vol. 3679, p. 87-98).
An inspection method for an exposure tool has been disclosed, in which an illumination distribution is measured on an aperture stop of the projection optical system or on a conjugate position of the aperture stop (Japanese Patent Laid Open No. 2928277).
The variation of the resist pattern dimension in the one-shot exposure region may result from various factors such as not only a local defect of the illumination optical system, but also an aberration and flare of the projection optical system, a dimensional error of the mask pattern on the photomask, a variation of a coated thickness of the resist film, nonuniformity of development, or the like. Accordingly, whether or not the illumination optical system of the exposure tool causes the variation of the resist pattern dimension may not be determined only by inspecting the transferred resist pattern for manufacturing a semiconductor device.
In the method disclosed in Japanese Patent Laid Open No. 2928277, it is necessary to install an illumination distribution detection unit in an appropriate position in the exposure tool. Thus, the configuration of the exposure tool maybe more complex. In addition, an exposure tool which does not have an illumination distribution detection unit cannot perform inspection.